A surface-treated copper foil has a surface treated by roughening, rust-proofing or the like in order to improve properties such as adhesive strength to a polyimide resin substrate for a copper-clad laminated sheet. A roughened surface of a surface-treated copper foil is bonded to a polyimide resin substrate to form a copper-clad laminated sheet. Then, a wiring pattern is formed by etching the surface-treated copper foil of the copper-clad laminated sheet, to provide a printed wiring board. On the printed wiring board are mounted electronic components, to provide a circuit board. In this process, electronic components are usually connected to wirings by soldering or wire bonding. Alternatively, an “anisotropic conductive film” (hereinafter, referred to as “ACF”) is sometimes employed.
An electronic component mounted on the circuit board must be connected to a wiring with adequate mounting strength for the prevention of the detachment of the component from the printed wiring board during use. The mounting strength is ensured by a soldering area in a soldering method or final resin sealing in a wire bonding method. Meanwhile, in an ACF mounting method, an ACF consisting of a resin component and conductive particles is used to electrically connect an electronic component to a wiring. However, the conductive particles contained in the ACF have no adhesion. Accordingly, due to the presence of the conductive particles, an area where an ACF, an electronic component, and a wiring are laminated may not have adequate adhesion strength. In a circuit board on which electronic components are mounted with an ACF, mounting strength of the electronic components depends on adhesion strength between “a surface of a polyimide resin substrate which is exposed by removing a surface-treated copper foil by etching” (hereinafter, referred to as a “resin base material surface”) and the ACF.
Patent Document 1 discloses a copper-clad laminated sheet comprising a copper foil as a conductive layer which is bonded to a flexible insulating layer and contains 0.1 to 10 at % of nickel on the surface bonded to the insulating layer; and a flexible printed wiring board comprising the same. Patent Document 1 explains that when electric connection is established using an ACF, the copper-clad laminated sheet and the flexible printed wiring board have excellent adhesiveness to the ACF and electric and physical connectivity with the ACF. The flexible printed wiring board is produced by selectively etching the copper foil such that nickel remains in an area where the insulating layer is exposed. Specifically, Examples of Patent Document 1 show that a two-layer flexible printed wiring board comprising a surface-treated copper foil which has a thickness of 12 μm and the surface bonded to the insulating layer with a surface roughness Ra of 1.0 μm exhibits excellent rust-prevention properties, etching properties, connectivity of a laser via hole, and adhesiveness to an ACF when the copper foil contains 0.1 to 10 at % of nickel on the surface bonded to the insulating layer as determined by XPS.
Patent Document 2 discloses a process for producing a metal wiring board comprising providing a metal foil-heat resistant resin substrate laminate wherein a metal foil is laminated on a polyimide resin substrate, and the surface of the metal foil to be bonded to the polyimide resin substrate is treated with at least one metal selected from the group consisting of Ni, Cr, Co, Zn, Sn and Mo, and alloys containing at least one of these metals; forming a metal wiring on the resin substrate; and then cleaning the surface of the resin substrate on which the metal wiring is formed with an etching solution capable of removing the metal used in the surface treatment so as to improve adhesiveness of the resin substrate surface.    Patent Document 1: Japanese Laid-open Patent Publication No. 2006-147662.    Patent Document 2: Japanese Laid-open Patent Publication No. 2007-134695.